More frequency bands are being used by mobile telephones due to an increasing number of mobile telephone users, more communication standards, expansion of services, etc. To prepare independent high frequency devices for such frequencies would undesirably lead to larger sizes and higher costs. Therefore, a high frequency device is shared. As an example, an antenna is shared by using a switch circuit for high frequency signals to connect the antenna to one of desired high frequency circuits.
Although High Electron Mobility Transistors (HEMTs) using compound semiconductors have been used in conventional switch elements of switch circuits for such high frequency signals, it is being studied that HEMTs is replaced with Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) formed on silicon substrates due to the need for lower costs and downsizing.
However, in a general MOSFET formed on a silicon (Si) substrate, parasitic capacitances between the Si substrate and source and drain electrodes is undesirably large and the power loss of the high frequency signal is undesirably high. Therefore, a semiconductor switch formed on a Silicon On Insulator (SOI) substrate has been proposed (for example, refer to JP-A 2005-515657).
Important characteristics necessary for semiconductor switches include insertion loss, isolation between switch ports, etc. Distortion characteristics also are important. A FET included in a semiconductor switch is a semiconductor device and therefore has nonlinearity; when power passes through the FET in ON state, ON distortion occurs due to the nonlinearity; and when power passes through the FET in OFF state, OFF distortion occurs.
When a high frequency signal is input to the semiconductor switch, it is necessary that a sufficiently low negative voltage is applied with respect to an input signal swing so as to reduce the OFF distortion occurring in a FET in the OFF state. However, in a FET formed on a silicon substrate, the negative voltage is too low, the OFF distortion occurring due to the nonlinearity of the FET undesirably increases.